For safety, advanced automobiles are now loaded with air bags which are normally folded and received in the steering wheel, but upon detection of shocks by collision, can be instantaneously inflated between the wheel and the driver to protect the driver from damages. The air bags are generally formed from nylon fabric by coating it with chloroprene rubber. These air bags undesirably have a short life since chloroprene rubber is less resistant against heat and weathering. The air bags are required to be flame retardant so that they are resistant against a fire and explosion which can break out upon collision of the car. Since chloroprene rubber is less flame retardant, prior art air bags formed from chloroprene rubber-coated fabric are further coated with a flame retardant silicone where they might be exposed to a blast.
Air bags formed from silicone rubber-coated fabric have the advantage of eliminated need for a flame retardant coating because well-known flame retardants can be blended in the silicone rubber to render the rubber itself flame retardant. Now air bag manufacturers are more interested in silicone rubber coating compositions for this reason and for their heat resistance and weather resistance.
A variety of silicone coating compositions have been developed for air bags. Various internal additives are incorporated in these compositions for imparting adhesion, flame retardancy, weather resistance, and heat resistance. As a result of concentrated efforts, currently available silicone rubber-base air bag coating compositions have superior characteristics to chloroprene rubber-base compositions.
However, conventional silicone coating compositions have a strike-through problem when applied to air bag base fabric of nylon or the like. In general, nylon base fabric on one surface is coated with a silicone coating composition using a knife coater, comma coater, gravure coater, offset roll coater or the like, often a knife coater and comma coater. On application, the silicone coating composition can penetrate or strike through the interstices of the nylon fabric. The struck-through composition on the fabric rear surface not only adversely affects the outer appearance of the fabric or air bag, but also reduces operating efficiency because the struck-through composition will adhere to backup rolls of the coating machine and if so, the coating machine must be stopped to remove the deposits. Since the struck-through portion is cured on the fabric rear surface at the same time as curing of the composition, if the coated fabric after curing is wound up and stored in roll form without special care, blocking would occur between the coating and the rear surface. The strike-through phenomenon occurs regardless of whether the silicone coating composition is applied with or without diluting it with solvent.
According to our investigations, the strike-through phenomenon takes place for the following reason associated with both the coating machine and the coating composition. When the coating composition is applied to air bag fabric by means of a coating machine, the composition must be adjusted to an adequate viscosity of 10,000 to 50,000 centistokes (cs) in order to form a thin coating with an optimum thickness of about 30 to 100 .mu.m, especially about 40 to 70 .mu.m in the case of silicone coatings. One typical prior art method to comply with this viscosity is to dilute a ultra-high molecular weight base polymer with a solvent to an adequate viscosity. Since the high molecular weight polymer generally has a wide distribution of molecular weight as synthesized, relatively low molecular weight polymer components contained therein tend to strike through the fabric when diluted with the solvent. Some polymers are designed to reduce the degree of dilution or to eliminate dilution prior to coating. However, polymers of such special design cure into coatings which are so weak that they will crack upon inflation of the air bag. Also, due to their reduced molecular weight, low-molecular weight components tend to penetrate through the fabric interstices, resulting in a strike-through phenomenon.
It is evident from above that the strike-through phenomenon occurs due to the low viscosity of the silicone coating composition upon coating and the relatively high content of low-molecular weight components in the silicone coating composition as a result of a wide molecular weight dispersion of the base polymer therein.